Devices based on Institute of Electrical and Electronics Engineers (IEEE) 802.11ad standard for the 60 Gigahertz (GHz) millimeter Wave (mmW) frequency band are being deployed in conjunction with IEEE 802.11 devices operating in frequencies below 6 GHz to provide improved user experience and expand the market for Wireless Local Area Networks (WLANs). Despite the enhanced capacity provided by the IEEE 802.11ad directional multi-gigabit devices, wireless LAN usages continue to grow and find new applications demanding additional capacity. For example, it is highly desirable to replace wired interfaces such as Ethernet, High-Definition Multimedia Interface (HDMI), Universal Serial Bus (USB), and DisplayPort whose speed can far exceed 10 Gigabits per second (Gbps) with wireless interfaces. In addition, there are other usages such as cellular offload, wireless docking, wireless display, and outdoor/indoor wireless backhaul. Therefore, there is a need to substantially increase the achievable throughput of IEEE 802.11ad devices and the overall capacity of IEEE 802.11 deployments, which is the main goal of the new IEEE 802.11ay amendment. In particular, Multiple-Input Multiple-Output (MIMO) transmission is considered as a key technology in IEEE 802.11ay to improve the data throughput of IEEE 802.11ad.
In some instances, an mmW communication system (e.g., IEEE 802.11ad) is operating with high number of antennas and very limited number of analog Radio Frequency (RF) chains. A large number of antennas is used to extend the communication range for recovering the high path loss while fewer analog RF chains are designed to reduce transmit, processing power, and hardware complexity. Due to the limited number of RF chains, the digital signal processor at Baseband (BB) frequency cannot apply individual fast-changing precoding weight or antenna weight to every antenna element to achieve the conventional fully digital preceding for MIMO transmissions at the transmitter or the conventional fully digital coherent combining at the receiver. Instead, only a slow-changing phase shift can be applied to each individual antenna element at the RF front-end to steer a beam towards the desired direction for each RF chain. Such a slow beam steering mechanism is commonly referred to as analog Beamforming (BF). On the other hand, the digital processor can apply a fast-changing preceding weight at BB to each RF chain. The application of such precoding weight per RF chain is referred to as baseband preceding. The combination of the analog BF and baseband preceding is commonly called hybrid beamforming.
Before any communication starts between two devices, the devices need to align their beam pointing angles towards each other. An efficient searching process to identify the best beam angle pair (a transmit beam and a receive beam) is therefore needed. This process is called beam training, which typically takes a significant amount of time to complete due to the numerous possible different combinations of transmit and receive beam directions to scan through. Due to the transmission of a single stream, beam selection criterion is clear, i.e., finding a beam pair to maximize the received signal power. In IEEE 802.11ay (enhanced version of IEEE 802.11ad), however, hybrid precoding will be used to multiplex several data streams together thereby improving throughput. As such, the current beam selection criterion (i.e., maximizing the received power) is no longer optimal.
Therefore, there is a need for a beam training protocol suitable for hybrid precoding, especially for multiple streams or different transmission modes.